This invention relates to an electric horn which generates sound by vibrating a diaphragm by electromagnetic means.
In a conventional electric horn, a diaphragm is vibrated by an electromagnet which is intermittently energized by intermittent supply of electric current, and thereby sound is generated. The diaphragm is urged back to its rest position by a return spring means of some sort.
The strength of the electromagnet should therefore be great enough not only to displace the air in a manner to make the sound, but also to bias the spring means. This means that a heavy and bulky electromagnet is needed.
Especially in a horn mounted in a ship or boat, which is usually mounted as high up as possible in order to increase the distance of propagation of the sound, lightness and compactness are particularly desired.
In such a conventional horn the sound produced by the diaphragm is usually amplified or directed by a horn trumpet into which the sound is directed at its base, and which provides a sort of resonance chamber which magnifies the sound and orients it in a particular direction. Such a horn operates best when it is particularly matched to the incoming sound; that is, when its dimensions are correctly tailored to the wavelength (not the frequency) of the sound produced by the diaphragm. Now the frequency of the sound produced by the diaphragm is determined by the oscillating frequency of the combination of the diaphragm, the return spring means, and the electromagnet, when, as is usually the case, a simple feedback system is used for energizing the electromagnet, of the conventional sort, wherein the return of the diaphragm to its position remote from the electromagnet by the spring re-energized the electromagnet. Even when a more sophisticated means of providing pulses to the electromagnet is used, this oscillating or "natural" frequency of the above defined combination is very influential in determining the output frequency of the sound. Therefore this sound cannot be of any frequency desired, but has a certain preferred frequency. Therefore, in practical design, according to this preferred frequency, the wavelength of the emitted sound is calculated, assuming a median value for the ambient air temperature and pressure, which determine the speed of sound and hence the frequency/wavelength relation of sound. However, of course, when the air temperature in particular varies from this assumed value, the loudness and quality of the sound emitted by the horn drop substantially, because the wavelength of the sound emitted at the preferred frequency of vibration of the diaphragm assembly changes, and accordingly is not well matched to the physical size of the horn trumpet.